Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets
Other comprehensive income - net unrealized losses on securities	$ 82.1	$ 105.8
Unearned premium reserve reduction	13.3	10.1
Compensation accruals	9.3	8.4
Impaired securities	2.0	2.0
Other comprehensive income - net funded status of benefit plans	2.0	2.3
Discounting of unpaid claims and claim expense tax reserves	2.7	2.8
Capital loss carryforward	0.8	0.0
Net operating loss carryforward	9.3	3.6
Intangibles	1.4	0.0
Postretirement benefits other than pensions	0.2	0.2
Total gross deferred tax assets	123.1	135.2
Deferred tax liabilities
Deferred policy acquisition costs	45.9	48.5
Life insurance future policy benefit reserve	46.4	42.4
Life insurance future policy benefit reserve (transitional rule)	4.3	6.4
Discounting of unpaid claims and claim expense tax reserves (transitional rule)	0.3	0.5
Investment related adjustments	44.8	29.9
Other comprehensive income - net reserve remeasurements	5.8	15.7
Other, net	9.0	0.6
Total gross deferred tax liabilities	156.5	144.0
Net deferred tax liability	$ 33.4	$ 8.8